It is known in the art to fuse glass to metal to form hermetically sealed electronic components. The reliability of such components is directly dependent on the integrity of the seal. There are two types of such seals employed in the industry which are commonly referred to as "matched seals" and "compression seals" with the former referring to seals formed from metals and glass having similar coefficients of thermal expansion. It has been estimated that as much as 80 percent of such seals utilized in the industry are the matched type.
Both types of seals are commonly formed by a three-step process including (1) degassing or decarburizing of the metal surface, (2) oxidation and (3) sealing, as disclosed in United Kingdom Patent No. 2,135,297A. Such conventional processes require that the metal be initially treated to prepare the surface by decarburizing and then oxidizing to form a thin oxide layer thereon. The seal is then produced on the treated surface by fusing the thin metal oxide layer with molten glass.
By way of example, the above-mentioned United Kingdom patent teaches that the first of the treatment steps, decarburization, is carried out in an atmosphere of 100 percent hydrogen with less than about 5.0 percent by volume of moisture, preferably at a H.sub.2 /H.sub.2 O volume ratio in excess of about 50. The second treatment step, i.e., oxidation, is carried out in an oxidizing atmosphere composed of a gaseous carrier containing an oxidizing agent such as water, carbon dioxide, nitrous oxide and the like and free hydrogen to reduce the oxygen content to thereby favor the formation of Fe.sub.3 O.sub.4 (page 2, line 52 to page 3, line 3).
The sealing step of the conventional process, as shown in the British patent, is carried out using a lean exo-gas or a N.sub.2 -based atmosphere. The temperature of each of the reaction steps can vary over a wide range with the preferred temperature range for each of the three steps being different. The British patent represents the state of the art prior to the present invention, i.e. the two treatment steps are regarded as critical in the formation of a good glass-to-metal seal.
In addition to the fact that each of the treatment and sealing steps of a conventional sealing process is preferably performed at different temperatures, each is carrier out in the presence of a different gaseous enviornment, which adds greatly to the time and cost of obtaining the desired product.
Applicants have discovered a unique process for producing matched seals in which highly effective glass-to-metal seals are obtained with untreated metal in a single step employing only a single set of process conditions.
In accordance with the present invention, the atmosphere in which sealing takes place is controlled within defined limits to prevent condensation in the furnace and to provide a sufficient thin layer of oxide on the metal for proper sealing of the glass to metal. The present process is also economical because it significantly reduces the amount of hydrogen needed to effect sealing. In addition, reduction of the glass-to-metal sealing process to a single step saves considerable time since downtime of the furnace to change gaseous environments is eliminated and parts handling is reduced by about two-thirds.
It is therefore an object of the invention to provide a single-step process for forming matched glass-to-metal seals with metal that has not been subjected to the conventional pretreatment steps of decarburization and oxidation.
It is a further object of the invention to provide a process for forming glass-to-metal seals which is less costly and time-consuming than conventional methods since only one heating step in a single atmosphere is required.
It is another object of the invention to obtain glass-to-metal seals which are strong, hermetic and have little, if any, entrained gas bubbles.